CD22 [BLCAM/1796] is a type 1 integral membrane glycoprotein with molecular weight of 130 to 140 kDa. It is expressed in both the cytoplasm and cell membrane of B-lymphocytes. CD22 antigen appears early in B-cell lymphocyte differentiation at approximately the same stage as the CD19 antigen (1). CD22 may be a useful marker for phenotyping mature leukemias, as CD22 membrane expression has been shown to be limited to the late differentiation stages between mature B cells (CD22+) and plasma cells (CD22-) (1,2). CD22 is also strongly expressed in hairy cell leukemia (3).